The anthraquinone process for making hydrogen peroxide is well known in the art to be a cyclic process in which quinones, dissolved in a solvent or mixture of solvents, are first hydrogenated in the presence of a hydrogenation catalyst to produce hydroquinones (hydrogenation stage or step). The hydroquinones are then oxidized to regenerate the original quinones with concomitant formation of hydrogen peroxide (oxidation stage or step). The hydrogen peroxide is removed, generally by water extraction (extraction stage or step) and the quinones are recycled.
The quinones, as used in this specification, include alkyl anthraquinones and alkyl tetrahydroanthraquinones. Thus, the resulting hydroquinones include the corresponding alkyl anthrahydroquinones and alkyl tetrahydroanthrahydroquinones. The hydrogenation catalysts and solvents are not critical to this invention and may be any which are disclosed in the art. The process may employ a slurry or fixed-bed hydrogenation catalyst.
The tetrahydroanthraquinones, which reduce to tetrahydroanthrahydroquinones, result from nuclear reduction of anthraquinones; that is, addition of hydrogen to the aromatic nucleus of the anthraquinone. Many have tried to reduce the formation of these "tetra" compounds because the tetrahydroanthrahydroquinones are slower in oxidizing back to tetrahydroquinones than the corresponding anthrahydroquinones are in oxidizing back to anthraquinones. See, for example, U.S. Pat. No. 2,730,533 in which a Raney nickel catalyst is treated with an amine, preferably an aliphatic amine, to prevent nuclear hydrogenation. Others have built processes around having a certain amount of the "tetra" compounds present with the anthraquinone. U.S. Pat. No. 2,995,424, for example, teaches a method of producing hydrogen peroxide from alkyl anthraquinones in which the working solution contains substantial amounts of tetrahydroanthraquinone. U.S. Pat. No. 2,995,424 employs a small amount of a water-soluble ionizable inorganic alkaline compound in the oxidation step as a catalyst to increase the rate of tetrahydroanthrahydroquinone oxidation.
The process of this invention will work regardless of "tetra" compound content in the working solution, but is particularly suited to the process in which "tetra" compounds are present. It is not directed at decreasing the products of nuclear hydrogenation. It is concerned with the other anthraquinone degradation species that result from overhydrogenation. Specifically these anthraquinone degradation species, also referred to as inert degradation compounds or species in this specification, are oxanthrones, the tautomers of anthrahydroquinones, and anthrones, the reduction products of oxanthrones. Further reductions also occur to make, in turn, anthranols and hydroanthracenes. Unlike the "tetra" compounds, these inert degradation species do not readily oxidize back to useful quinones and do not produce hydrogen peroxide in the normal cyclic operation of the process. As the degradation products become further reduced, the more difficult they become to reoxidize. The result is loss of valuable quinones from the process and a substantial cost penalty of operation. The inert degradation species must be removed and replaced with quinones if the desired hydrogen-peroxide-synthesizing capacity is to be maintained.
Since formation of inert degradation species is costly, much effort has been directed at avoiding their formation. Catalysts have been treated to make them more selective and substances have been added to the working solution in attempts to limit formation. Also, attempts have been made to convert the degraded compounds into useful compounds.
U.S. Pat. No. 3,098,714 teaches addition of water-soluble alkaline-reacting substances to the working solution, in the hydrogenation stage to increase the activity of the hydrogenation catalyst with a resulting reduction in by-product formation. The additive is expelled in the reduction or oxidation stage or lost during water extraction.
U.S. Pat. No. 3,307,909 teaches the admixture of a catalyst-free, solid particulate inorganic alkaline material which is insoluble in the working solvent with the hydrogenation catalyst to reduce inert degradation compound formation. This additive was removed with the hydrogenation catalyst prior to the oxidation stage.
British Pat. No. 795,272 teaches addition of an alkaline compound, particularly an organic amine, having a dissociation constant greater than 1.times.10.sup.-10 to the organic solvent in the hydrogenation stage or the substitution of it for the organic solvent to reduce the rate of formation of inert degradation compounds. The additive must not be subject to hydrogenation under conditions of operation, must be a solvent for the quinones or be soluble in the organic solvent and preferably should be essentially immiscible with or insoluble in water.
U.S. Pat. No. 3,295,928 teaches regeneration of degraded working solution by the combined treatment of the solution with an organic or inorganic base which should be water soluble and an oxidizing agent after the hydrogen peroxide is extracted but before the solution is hydrogenated.
The process of this invention is not intended to prevent formation of inert degradation compounds as such. It is intended to minimize the buildup of inert degradation species, particularly oxanthrones and anthrones, by introducing an additive to catalyze their reoxidation to useful quinones. Preferably the additive should be such that it can be introduced at any stage of the anthraquinone cyclic process but not readily lost from the system during any stage. It should not contaminate the hydrogen peroxide product. To meet these objectives, the additive should be soluble in the working solution and resistant to hydrogenation in the presence of the hydrogenation catalyst. Additionally, it should be stable to oxidation by air or hydrogen peroxide and should be insoluble in and nonreactive with water and hydrogen peroxide, particularly at such adverse operating conditions as experienced in typical plant extraction units (i.e., at temperatures up to about 60.degree. C. and hydrogen peroxide concentrations up to 35 weight percent). The process of this invention comprises the addition of aromatic tertiary amines which meet these conditions but still are basic enough to maintain a sufficient catalytic activity level to reoxidize the inert quinone-degradation species to useful quinones. Further, the addition of aromatic tertiary amines has shown other benefits. Specifically, they have been found to contribute to a more even level of noble-metal-hydrogenation-catalyst activity and selectivity. They have been found to be beneficial in regenerating the activity of catalyst which has become deactivated due to process upsets. The presence of the aromatic tertiary amine also enables a higher level of hydrogen peroxide recycle from the extractor than could previously have been tolerated. Without the aromatic tertiary amine, the noble-metal catalysts are prone to loss of activity and selectivity as a result of small increases of hydrogen peroxide in recycled working solution.